Fertiliser for pots and gardens: what to use, when, and how much

Stand in front of the fertiliser shelf at any nursery or co-op and it is easy to feel outgunned. Bags of 2:3:2 and 3:1:5, boxes of bone meal, bottles of seaweed, blue crystals, brown granules, pellets that promise six months of feeding. Most of them are perfectly good products. The trouble is that the labels assume you already know what your plants are asking for, and most of us are guessing.

We have fed a lot of pots over the years, and the honest summary is this: most garden plants want less fertiliser than the packaging suggests, and plants in containers want it more often but in smaller doses. Get those two ideas straight and the rest of the shelf becomes a set of tools rather than a puzzle. Here is how we think about it, in South African terms, with our spelling and our seasons.

NPK in plain words

Every fertiliser bag carries three numbers, and in South Africa they are written as a ratio like 2:3:2 or 3:1:5, usually followed by a figure in brackets such as (22) or (26). The letters behind the numbers are N, P and K. Nitrogen drives leaves and green growth, which is why lawn feeds are heavy on it. Phosphorus builds roots and gets flowering and fruiting started. Potassium is the toughness nutrient, thickening cell walls, improving fruit quality and helping a plant cope with heat, cold and drought. The number in brackets is the total percentage of those three nutrients in the bag, so 2:3:2 (22) is 22 percent nutrient and 78 percent carrier.

That makes the labels useful once you know the code. A 2:3:2 is the gentle general-purpose feed for beds and new plantings. A 3:1:5 leans towards flowers and fruit, so it suits roses, citrus in pots and tomatoes. High first numbers like 5:1:5 or straight LAN push leafy growth and belong on lawns and leafy vegetables, not on a pot of lavender. The RHS guide to fertiliser types is a good reference for what each nutrient does once it is in the plant.

Organic or synthetic, and why we use both

Organic fertilisers are the ones that were recently alive. Compost, kraal manure, bone meal, blood meal, seaweed and kelp all release nutrients slowly as soil life breaks them down, and they improve the soil while they do it. Their numbers look feeble next to a synthetic bag, but that is the point: they are almost impossible to overdo, and they feed the fungi and bacteria that plants depend on. The catch is speed. A plant that is starving today will not be rescued by bone meal this week. Manure also needs to be properly rotted, because fresh kraal manure is salty and hot enough to scorch roots.

Synthetic fertilisers are precise and fast. The nutrients are already in a form roots can drink, so a yellowing plant greens up within days. They do nothing for soil structure, they wash away with heavy watering, and a heavy hand burns leaves and roots. Our approach is to build the soil with organic matter first and reach for the synthetic bag only when a plant needs a targeted push. A few handfuls of bark compost worked into the top of a bed or pot each spring does more long-term good than any granule, and our article on the difference between compost, fertiliser and soil explains why they are not interchangeable.

Slow-release granules or liquid feed?

Granular fertilisers sit in the soil and dissolve over weeks. Coated slow-release types, the little round pellets, meter out nutrients over three to six months depending on temperature, which makes them the low-effort option for pots you tend to forget. Uncoated granules like 2:3:2 release faster and need reapplying every six to eight weeks in the growing season. Either way, scratch them lightly into the surface, keep them off stems and leaves, and water them in straight away.

Liquid feeds are diluted into the watering can and reach the roots immediately. They are the quickest way to correct a deficiency and the easiest to control in containers, because you can adjust the dose to the plant in front of you. Seaweed extract sits in this group too, more as a tonic than a meal, and plants that have been through a hail storm or a transplant appreciate it. For most pots we run both: a slow-release granule in spring as the baseline, with a half-strength liquid feed every fortnight for the heavy feeders through summer.

Why pots need a different rhythm to beds

A garden bed has metres of soil beneath it, a living reserve of nutrients and a web of fungi ferrying minerals to roots. A pot has 15 or 30 litres of bark-based mix and a hole in the bottom. Every time you water thoroughly, and you should water thoroughly, some of the nutrients rinse out through that hole. Potting soil is also mostly composted bark, which is brilliant for drainage and air but holds very little food of its own. Within six to eight weeks of planting, a container has usually used or lost most of what came in the bag.

So the rule for containers is little and often. Rather than one big dose, feed at half the recommended strength twice as frequently, and stop when growth stops. On the Highveld that means feeding from September, when the first warm rains arrive, through to the end of March, and nothing at all through the frosty months when the plant is not growing and cannot use it. In frost-free coastal gardens you can carry on lightly into winter for plants that keep growing. Refresh the top few centimetres of a pot with fresh mix and compost every spring; our guide to the best type of soil for plants covers what belongs in the pot to begin with, and our growing food in pots guide gives feeding schedules for the hungriest crops.

Who eats a lot and who barely eats

Not every plant wants the same menu. Tomatoes, peppers, brinjals, bananas, dahlias, roses and citrus are heavy feeders that will show hunger within weeks in a container. Leafy greens like spinach and lettuce want nitrogen and want it steadily. University extension services put clear numbers on this: the University of New Hampshire's vegetable fertilising guide is a sensible model for how much and how often, even if the products have different names here.

At the other end, most indigenous shrubs, succulents, aloes, lavender, rosemary and proteas evolved on thin, poor soils and resent being fed. Give them a well-drained mix and a light dusting of compost, and that is enough. Proteas and other fynbos in particular react badly to phosphorus, so keep bone meal and 2:3:2 well away from them. If you are not sure which camp a plant sits in, our article on the importance of soil health is a useful place to start, because good soil covers for most feeding mistakes in both directions.

Reading the leaves: too little and too much

Plants tell you what they lack if you look at which leaves are complaining. Pale, yellowing older leaves at the bottom of the plant with the new growth still green usually mean nitrogen is short. A purplish tinge on leaves and stems, especially in cool spring weather, points to phosphorus. Brown, scorched-looking leaf edges and weak stems that flop are a potassium sign. Yellow young leaves with the veins still dark green are almost always iron, and in Gauteng that is often a pH problem in alkaline soil rather than a real shortage. The RHS page on nutrient deficiencies has photographs that make the diagnosis much easier.

Over-feeding looks different and is more common in pots than under-feeding. A white crust on the soil surface or around the drainage hole means salts have built up. Brown, crispy leaf tips on a plant with wet soil, wilting despite regular watering, or a sudden flush of soft, lush growth that aphids move onto within days are all signs of too much. The fix is to flush the pot: stand it somewhere it can drain and run clean water through until you have poured in roughly three times the pot's volume, then skip feeding for a month. Oregon State's guide to fertilising vegetables, fruits and ornamentals puts it plainly: more is not better, and the damage from excess is harder to undo than the damage from a little hunger.

What not to do

Never feed a dry plant. Fertiliser is a salt, and salt drawn into parched roots burns them; water thoroughly first, wait an hour, then feed. Never feed a sick or stressed plant hoping to perk it up. A plant that is wilting from root rot, a pest attack or heat shock cannot use the food, and the extra salt makes it worse. Sort out the cause first, and our quick guide to potted plant pests helps rule out the usual suspects.

Leave newly potted plants alone for four to six weeks. Fresh bark potting soil already has enough to get roots settling, and feeding at that stage just pushes weak top growth. Do not feed in the heat of a summer afternoon, keep granules off leaves and stems, and resist the urge to double a dose because a plant looks slow. Do not put lawn fertiliser on flowering pots either; the nitrogen gives you a leafy plant that forgets to bloom. When in doubt, use less. A hungry plant is easy to fix. A burnt one is not.

Fertiliser is only the last layer of the picture. The pot matters, the drainage matters, and the soil you start with matters most of all, because compost-rich mix holds nutrients between feeds and buffers your mistakes. Our soils and compost range covers the base layer, from bark compost for beds to potting mixes for the containers, so the feeding you do on top actually stays where the roots can find it.

Frequently asked questions

What do the numbers on a fertiliser bag mean?

They show the ratio of nitrogen, phosphorus and potassium. In South Africa a label like 2:3:2 (22) means two parts nitrogen to three parts phosphorus to two parts potassium, with the bracketed figure giving the total percentage of those nutrients in the bag.

How often should I fertilise plants in pots?

During the growing season, roughly September to March on the Highveld, feed containers little and often: a half-strength liquid feed every two weeks or a slow-release granule every three months. Stop feeding in winter when growth slows.

Is organic fertiliser better than synthetic?

Organic fertilisers like compost, manure and bone meal feed the soil slowly and are hard to overdo, while synthetic fertilisers act fast and precisely but can burn plants and do nothing for soil structure. Most gardens do best with an organic base and occasional targeted synthetic feeding.

What are the signs of over-fertilising?

A white salt crust on the soil, brown crispy leaf tips despite wet soil, wilting after watering and a sudden flush of soft growth that attracts aphids. Flush the pot with three times its volume of clean water and stop feeding for a month.

Should I fertilise a plant that is wilting or sick?

No. Water it thoroughly first if it is dry, and deal with pests, rot or heat stress before feeding. Fertiliser is a salt and a stressed plant cannot use it, so feeding usually makes the problem worse.